1. Field of the Invention
The present invention relates to electrophoretic particles for use in an electrophoretic display apparatus utilizing charged particles in a medium being moved by application of a voltage, a process for production of the electrophoretic particles, and an electrophoretic display device using the electrophoretic particles.
2. Related Background Art
The electrophoretic phenomenon is a phenomenon in which when certain particles are suspended in a medium (dispersing medium), the particles are electrically charged, and when an electric field is applied to the charged particles, they move (make a migration) through the dispersion medium to an electrode having an opposite charge. Electrophoretic particles for use in an electrophoretic display apparatus using such a phenomenon include, for example, inorganic pigments such as titanium oxide, zinc oxide, zirconium oxide, iron oxide, aluminum oxide, cadmium selenide, carbon black, barium sulfate, lead chromate, zinc sulfide and cadmium sulfide, and organic pigments such as phthalocyanine blue, phthalocyanine green, Hansa yellow, Watching red and Dialyride yellow. For conventional electrophoretic particles, however, there have been cases where electrophoretic particles dispersed in the dispersion medium coagulate together, or electrophoretic particles gradually settle out over time due to a difference in specific gravity between the electrophoretic particle and the dispersion medium, or the electrophoretic particle is not charged enough to make a response to application of a voltage sufficiently, and a phenomenon of irreversible adsorption to an electrode plate occurs.
In order to solve these problems, it has been proposed that the specific gravity of the electrophoretic particle is made substantially equal to that of the dispersion medium by covering the electrophoretic particle with resin (Japanese Patent Application Laid-Open No. 48-31097).
In addition, it has been proposed that small particles of titanium dioxide as electrophoretic particles are covered with silicon resin, thereby giving a large amount of spontaneous charge to the electrophoretic particles, and these electrophoretic particles are used in an electrophoretic device with dyes coexisting in the dispersion medium (Japanese Patent Application Laid-Open No. 02-189525). In addition, it has been proposed that small particles of titanium dioxide as electrophoretic particles are covered with polyethylene resin, thereby giving a large amount of spontaneous charge to the electrophoretic particles, and these electrophoretic particles are used in an electrophoretic device with dyes coexisting in the dispersion medium (Japanese Patent Application Laid-Open No. 09-211499). In addition, it has been proposed that electrophoretic particles are treated with a titanate based coupling agent and sorbitan fatty ester (Japanese Patent Application Laid-Open No. 02-284128).
In addition, it has been proposed that an functional group is introduced on the surface of the electrophoretic particle, and this functional group is subjected to a graft reaction with a polymer, thereby modifying the surface of the electrophoretic particle (Japanese Patent Application Laid-Open No. 05-173193).
The electrophoretic particle covered with the above described silicon resin or polyethylene resin has a specific gravity substantially equal to the dispersion medium, thus making it possible to prevent the settling of electrophoretic particles in the dispersion medium and to increase the amount of charge, but it is necessary to disperse electrophoretic particles in a heated and dissolved resin and finely grind the electrophoretic particles to sizes suitable for the electrophoretic display device after they are cooled and cured in their production process, thus bringing about a disadvantage that enormous amounts of energy and time are required. Also, in the electrophoretic device with dyes coexisting in the dispersion medium, there is a disadvantage that the resin with which electrophoretic particles are covered tends to be smeared with dyes and the like in the dispersion medium, and therefore a contrast between the electrophoretic particle and the dispersion medium may be reduced.
In addition, introduction of a functional group on the surface of the electrophoretic particle to treat the electrophoretic particle with a titanate based coupling agent and sorbitan fatty ester, and graft reaction of the electrophoretic particle with a polymer are effective measures for surface modification, but these measures have a disadvantage that functional groups that can be introduced depending on a pigment to be used are limited, and introduction of functional groups on the surface depending on a variety of pigments may be complicated and lacking in general versatility.
The present invention has been made in view of these circumstances, and its object is to provide electrophoretic particles for electrophoretic display having excellent dispersibility and dispersion stability with time for an insulating medium and prevented from coagulating and settling out, a process for production of the electrophoretic particles that is versatile for pigments to be used in order to correspond to a full-color display, and an electrophoretic display device using the electrophoretic particles that has an excellent memory property and is highly reliable.